The icy giant worlds Uranus and Neptune are the least studied planets in the Solar System. Of all the space probes only Voyager 2 visited them, and their great distance from the Sun (and therefore Earth) makes them difficult to study with ground-based telescopes. As a result many aspects of the planets are mysterious, including the strong winds in their atmospheres.

A new study of data from Voyager 2 and the Hubble Space Telescope may have demonstrated that the weather on Uranus and Neptune is confined to a relatively thin layer of atmosphere. Yohai Kaspi and colleagues analyzed variations in the planets' gravitational fields, which are affected in a small way by atmospheric fluctuations. They compared various models of both the atmosphere and interior and determined the region containing the strong winds comprised a very tiny fraction of the planets' mass: 0.2 percent or less. However, they stressed that the ultimate cause of the winds probably lies in the planets' warm interiors, especially on Neptune.

As with Jupiter and Saturn the atmospheres on Uranus and Neptune separate into zones, wide bands where the prevailing winds blow in the same direction. On all four worlds the zones alternate directions with the winds blowing the same direction as the planets' rotation in one zone and in the opposite direction in the neighboring zone. Neptune's measured wind velocities can top -300 meters/second (-670 mph, where the negative indicates a direction opposite to the planet's rotation) in the equatorial zone while Uranus achieves the still-impressive but more sedate 200 m/s at tropical latitudes.

North and South

A planet's north pole is defined by its direction of rotation: if you look at a planet from "above" its north pole it will appear to rotate counterclockwise. You can also determine this by curling the fingers of your right hand in the direction of the planet's rotation; your thumb will point north. Because Venus rotates in the opposite direction—retrograde—from Earth and most of the other planets its north pole is "down" relative to other worlds. Uranus' axis is tilted 98° so it rotates in the retrograde direction as well as being nearly on its side.

Neptune is also surprisingly warm, producing about 1.6 times as much heat from its interior as it receives from the Sun. (Uranus only puts out about 6 percent as much heat as it receives from the Sun, for comparison.) The role of the internal heat source in the rapid winds is uncertain, but based on the lack of other drivers, an internal heat source must be involved on some level.

The gravitational field of a planet depends on its mass, but it also varies based on the shape, rotation, and density profile of the body. In particular, if the windy turbulent portion of the atmosphere is thick, it should contribute to the overall gravitational field in a distinctive way. Voyager and Hubble data provided a good map of the wind patterns on Uranus and Neptune; the authors of the current study used those to model how the gravitational field would be affected if the windy part of the atmosphere were thick or thin.

The results were surprising: the best fit to the data showed no contribution to the gravitational field from the part of the atmosphere with the planets' weather. That enabled the researchers to put an upper limit on the thickness and mass of the windy portion of the atmosphere: 1000 kilometers thick and 0.2 percent of the planets' total mass. 1000 kilometers may seem thick, but both Neptune and Uranus are roughly 25,000 kilometers in radius, with the atmosphere comprising a significant fraction of that total.

The researchers stressed these are upper limits: the thickness and mass of the weather-bearing atmospheric layer could very well be less. In the absence even of projected probes to Uranus and Neptune close-up observations of the planets are unlikely to happen anytime in the next few decades. However, it would be nice to have a Cassini-style mission to one or both planets to understand their mysterious structures better.

Matthew Nice article. I am amazed at the techniques scientists use to tease information out of data! The contribution of various parts of the atmosphere to the gravitational field being used to measure the thickness of the high wind regions. Wow.

Hmm, rocky core... sounds rather like a solid surface to me. Perhaps a better way of putting it is that they don't have a solid-atmosphere interface. Basically, these sound like the methane/ammonia equivalent of extremely massive "water" worlds, where water -> water + other liquids.

Matthew Nice article. I am amazed at the techniques scientists use to tease information out of data! The contribution of various parts of the atmosphere to the gravitational field being used to measure the thickness of the high wind regions. Wow.

Or the entire technique is fundamentally flawed. Their result is basically: there's a zero thickness region for the winds so we'll say it's within the error band because, well ... umm, you know we've seen clouds moving really fast. Have we seen this technique actually work on another planet like Jupiter or Saturn?

Methane can come from other sources such as volcanic activity, however if I remember correctly, methane breaks down pretty quickly in the presence of oxygen.

Don't know where all the methane on Neptune or Uranus came from to begin with though, I am sure there is an easy answer that I don't know.

The existence of methane such as on Titan, is evidence of amino acids, the building blocks of life.But it doesn't indicate that there is life, especially on these cold worlds, as these harsh climates slow down the formation of life.What NASA or the ESA needs to do is send more landing probes like the one on titan, and try to make them cheap, and fully automated.The probes could also boyently float in the atmosphere.Though, they'd have to be RITG powered. (Kind of short on the specific plutonium atm).All said, I am very glad to see some new highlights on the last two uncharted planets, even if it's speculated data from old evidence.

I'm really curious what it would look like if you were able to get a camera into the atmosphere (that would need to be almost indestructible). What would it look like? Would it just sink down through most of the planet? Or would it be densely compressed enough at a certain point that you could call it solid?

Hydrogen, helium, and methane are highly flammable materials. A spark of electricity from outer space could ignited the whole planet into a fire ball. I wonder what happens to a planet as close as Earth when they catch on fire is unimaginable. Would Earth be blown out of its orbit and head to the Sun, or head the opposite direction away from the Sun? The unimaginable is we are either freeze to death or burn to death, with no other choice. I am afraid.

Hydrogen, helium, and methane are highly flammable materials. A spark of electricity from outer space could ignited the whole planet into a fire ball. I wonder what happens to a planet as close as Earth when they catch on fire is unimaginable. Would Earth be blown out of its orbit and head to the Sun, or head the opposite direction away from the Sun? The unimaginable is we are either freeze to death or burn to death, with no other choice. I am afraid.

'A spark of electricity from outer space could ignited the whole planet into a fire ball'Please remember your fire triangle...

'I wonder what happens to a planet as close as Earth when they catch on fire is unimaginable.'We have over 2.5 billion kilometres of the universes finest insulator. Even if you were to smash a sufficient body of oxygen in to the planet and ignite the whole thing we wouldnt notice a thing.

An educated guess corrections would be welcome,*The ridiculously sized container of oxygen required to do this (and make no mistake, it would require a truly staggering amount.) releases its contents which are mixed in to the planet. A scientist ignites it and the atmosphere turns to a fire storm.

Its important to note here that this isnt strictly an explosion. In order to produce an explosion the entire planet would have to be encased in a container so it could build up pressure. Without that container its just bog standard burning. I should think youd be fine in orbit let alone from the distance of the Earth.

Anyway, a problem for the aesthetics is that Hydrogen makes up most of the atmosphere and burns with a very pale blue colour. From Earth youd probably struggle to see a difference with visible light. Helium of course doesnt react at all and would just hamper the whole process and there is very little methane to speak of.Disappointing so far.

The fire storm would take a bit of time to burn away but would ultimately leave a whole bunch of Helium and a staggering amount of water which, spread out so thinly, would fairly rapidly radiate its heat away. The blue planets would turn a much paler shade of blue close to white as they become the biggest snowballs in the Solar system. (It should be a brilliant bright near white colour in contrast to Europa which is quite dirty coloured. This is because it would have very few impurities upon creation.)

* Note this is all based on two factors, that the oxygen is mixed perfectly and not ignited too soon and that the oxygen and hydrogen are dense enough to sustain a reaction. I imagine that mixing in the oxygen would be impossible without accidental ignition from the planet or external factors.I also should think that a good chunk of the atmosphere would remain due to being too sparse to keep the burn on.

I wonder what happens to a planet as close as Earth when they catch on fire is unimaginable. Would Earth be blown out of its orbit and head to the Sun, or head the opposite direction away from the Sun? The unimaginable is we are either freeze to death or burn to death, with no other choice. I am afraid.

What happens to a planet as close as Earth (ie. roughly 4,300,000,000 km at closest point)? Not much. If the atmosphere could catch fire as you describe, it's still a chemical reaction rather than the fusion nuclear reaction of a star igniting (and hence orders of magnitude less powerful).

Methane is a simple molecule and is widely dispersed in gas clouds. Oxygen on the other hand is highly reactive and is not present in anything like our atmosphere's concentration (~20%) as it combines to form other more stable compounds (CO2, H20) in reducing conditions.

The early Earth is thought to have had lots of carbon dioxide, water vapor, nitrogen, and some other gases including methane. This is a reducing atmosphere, following the oxygenation of the atmosphere by blue green algae it was changed to an oxidating atmosphere.

I'm really curious what it would look like if you were able to get a camera into the atmosphere (that would need to be almost indestructible). What would it look like? Would it just sink down through most of the planet? Or would it be densely compressed enough at a certain point that you could call it solid?

More likely it would a gradual change from a gas to a liquid. This is postulated for all the gas giants. In the case of Uranus it would be a highly charged "ocean" of water, amonia, and methane which could be driving the high wind speeds (lack of solid surface, internal heat source, etc.). I think Uranus is probably the second most interesting object in the solar system (behind Titan). I'd like to have been around when the colision occured that knocked it so off kilter.